1999 Ford F-150 HELP!

BUTTERFLY95

1999 FORD F-150

Electrical problem
1999 Ford F150 6 cyl Automatic 135000 miles

My husband and I have exited our Ford F150 only to find out when we have come to get into it again the stereo and the idiot lights have been on. This is with no key in the ignition. This is so strange. Needless to say the battery was drained those times. Now when we go somewhere my husband disconnects the battery cables so it will start. This is getting old. We took it to our mechanic and he could'nt tell what the problem was. We don't have the money to have another mechanic/s experiment with what could be the problem. Has any one had this problem and if so what was it? You're time and info is greatly appreciated. Thank you.